The force exerted on a material under light irradiation is called the radiation force. The force is used in optical manipulation to control the 3-dimensional position and dynamic motion of a fine object. A specific example is optical manipulation of micrometer-sized fine objects floating in a fluid medium. It is expected that the force will be exerted also in nanotechnology.
It was difficult, however, to apply the radiation force-based optical manipulation in the field of nanotechnology with conventional techniques. A specific cause which makes it difficult to optically manipulate nanosize objects is insufficient radiant force. Nanotechnology deals with a nanomaterial (objects, particles, or structures, each of a few hundreds of nanometers or smaller). Non-metal nanomaterials, under ordinary conditions, show too small induced polarization to produce sufficient radiation force for the control of the motion of the nanometer-sized objects by light irradiation.
On the other hand, it is known that shining a laser beam at a frequency resonant with an electronic excitation level of the target material (resonant light) enhances induced polarization, hence achieving strong radiation force. The principles are utilized in laser cooling and capture of atoms.
The nanomaterial is also known to exhibit characteristics which are derived from the fact that its quantum-mechanical properties change with its size (dimensions), shape, internal structure, quality, etc. unlike micrometer-sized materials and atoms.
The inventors of the present invention, in view of this knowledge, have conducted theoretical studies of the radiation force exerted on nanosize objects under resonant light irradiation, as well as its quantum-mechanical effects, which has led to the following findings:
(1) Advantages in exploiting electronic resonance effects increase tremendously with decreasing size. For example, the force exerted on about 10-nm objects under certain conditions increases, due to the resonance effects, at least four orders of magnitude in comparison to the case without the resonance effects. Under a certain condition in the presence of resonance effects, weak incident light which would produce only a linear response can induce a force greater than gravity by a few orders of magnitude.
(2) Nano objects of about a few tens of nanometers in size go through a coherent scattering process in which excitation energy dissipates through radiation more quickly than in a thermal absorption process. Manipulation entailing almost no heat generation may become possible by using the process.
(3) Peak positions of force in a frequency spectrum shift highly sensitively in response to changes in size on the order of nanometers due to quantum size effects.
Accordingly, the inventors of the present invention, in view of this knowledge, proposed a novel optical manipulation technique which exploits the fact that the radiation force, induced when nanosize objects are illuminated with resonant light, changes with the quantum-mechanical characteristics of individual nanosize objects, in order to selectively manipulate nanosize objects of particular nature (see non-patent document 1 and patent document 1).
A so-called “quantum dot” is an example of an optically manipulable nanosize object. The quantum dot is frequently referred to as the artificial atom because its electronic excitation levels are discrete like those of an atom.
Although current quantum dot research is still in its fundamental stage, it is known that the quantum dot has high quantum efficiency and is easy to use as a device because the dot can be formed of a semiconductor or similar unique material. The quantum dot is expected to find applications in a variety of fields, such as high efficiency light emitting devices, high speed optical communications, quantum communications, and biotechnology. Recently, there are a lot of studies, especially, of electrical and optical properties of semiconductor quantum dots in which an electron system is confined. Focus is not only on single quantum dots, but also on quantum-mechanical coupling between a plurality of quantum dots.
For example, coherent bonding and antibonding states of electrons in an excited state are observed between quantum dot pairs. Such a pair of quantum dots is termed an artificial molecule or quantum dot molecule (see non-patent documents 2, 3). In the artificial molecule, the electrons confined in individual quantum dots are quantum-mechanically entangled. Thus, if a device is fabricated which contains large numbers of arrays of the artificial molecules, that device will likely be developed into quantum computers and find other applications.
There are also attempts to control energy transfer between quantum dots (see non-patent document 4). The energy transfer control will enable efficient energy transfer and may contribute to solution to energy problems.
The following is the list of the documents mentioned above:
Patent Document 1: Japanese Unexamined Patent Publication (Tokukai) 2003-200399 (published Jul. 15, 2003).
Non-patent Document 1: T. Iida, H. Ishihara, Phys. Rev. Lett., Vol. 90, 057403, pp. 1-4 (Feb. 7, 2003).
Non-patent Document 2: M. Bayer, P. Hawrylak, K. Hinzer, S. Fafarad, M. Korkusinsi, Z. R. Wasilewski, O. Stern, A. Forchel, Science, Vol. 291, 451 (2001).
Non-patent Document 3: T. H. Oosterkamp, T. Fujisawa, W. G. van der Wiel, K. Ishibashi, R. V. Hijman, S. Tarucha, L. P. Kouwenhoven, Nature, Vol. 395, pp. 873-876 (1998).
Non-patent Document 4: S. A. Crooker, J. A. Hollingsworth, S. Tretiak, V. I. Klimov, Phys. Rev. Lett., Vol. 89, 186802, pp. 1-4 (Oct. 24, 2002).
These conventional techniques fall short of efficient manipulation of more than one nanosize objects with high degrees of freedom.
Specifically, for example, non-patent document 2 observes a quantum-mechanical bonding state of quantum dots formed in a layered structure of semiconductors by self-assembly. The technique utilizes effects of distortion which is in turn caused by different lattice constants of different semiconductors stacked by MBE or a similar method. The technique is capable of producing a large number of quantum dots at a time. Nevertheless, given a set of semiconductors, the positions of the dots are automatically determined.
Non-patent document 3 subjects layers of gallium arsenide (GaAs) and aluminum gallium arsenide (AlGaAs) to an insulation process by focused ion beam implantation. Quantum dots of 100 nm are made by applying voltage through a Schottky gate. It is presumably difficult to produce smaller quantum dots or in large quantities by this technique.
In the techniques described in non-patent documents 2, 3 and other literature, the quantum dots are fixed in position in the semiconductor stack. The quantum dots are not mechanically manipulable even by illuminating the quantum dots with light to excite the bonding or antibonding energy for electrons in an excited state. Once the quantum dots are formed, their positions are no longer freely controllable.
Non-patent document 4 deposits and fixes colloidal cadmium selenide (CdSe) quantum dots dispersed in an organic solvent on a glass substrate. The technique indeed mechanically manipulates the quantum dots in the organic solvent. Since the quantum dots are ultimately fixed to the substrate, however, it is difficult to control the diameters of the quantum dots, distances between the quantum dots, and other determining factors in energy transfer.
In contrast, by using the techniques disclosed in non-patent document 1 and patent document 1 developed by the inventors of the present invention, it is possible to form large numbers of nanosize objects with selected specific quantum-mechanical properties in free space, and to control their motion. The documents however left unanswered for future discussion the question of details of how to control the motion of the nanosize objects by means of light-induced force between the objects.
The present invention, conceived in view of the above problems, has an objective of providing a collective manipulation technique for nanosize objects, such as quantum dots or quantum dot pairs.